Conventional mineral oil based lubricants are produced by a separative sequence carried out in the lube refinery that includes fractionation of a paraffinic crude under atmospheric pressure followed by fractionation under vacuum to produce distillate fractions, such as neutral oils, and a residual fraction which, after deasphalting and severe solvent treatment, may also be used as a lubricant base stock, usually referred as a bright stock. Neutral oils, after solvent extraction to remove low viscosity index (VI) components, are conventionally subjected to dewaxing, either by solvent or catalytic dewaxing processes, to obtain a desired pour point, after which the dewaxed lube stock may be hydrofinished to improve stability and remove color bodies. This conventional technique relies upon the selection and use of crude stocks, usually of a paraffinic character, which produce the desired lube fractions of the desired qualities in adequate amounts.
The range of permissible crude sources for lubricant production may be extended by a lube hydrocracking process that is capable of utilizing crude stocks of marginal quality, often having a higher aromatic content than the best paraffinic crudes. The lube hydrocracking process generally includes an initial hydrocracking step carried out under high pressure in the presence of a bifunctional catalyst that effects partial saturation and ring opening of the aromatic components present in the feed. The hydrocracked product is then subjected to dewaxing in order to reach the target pour point.
Conventional mineral oil lubricants have been challenged to match the lubrication demands of modern automotive engines. Current trends in the design of automotive engines have yielded higher operating temperatures, requiring higher quality lubricants. This has resulted in the need for lubricants having higher viscosity indices. High VI values have conventionally been attained by the use of VI improvers, such as polyacrylates, but there is a limit to the degree of improvement which may be effected in this manner. In addition, VI improvers tend to undergo degradation under the effects of high temperatures and high shear rates, with more stressing conditions producing even faster degradation of oils having significant amounts of VI improvers.
Synthetic lubricants produced by the polymerization of alpha olefins in the presence of certain catalysts have been shown to possess excellent VI characteristics. However, historically, they have been expensive to produce by conventional synthetic procedures and usually require expensive starting materials.
Another approach to the production of high VI oils has been to subject petroleum waxes to severe hydrotreatment over amorphous lube hydrocracking catalysts, followed by dewaxing to a target pour point. In processes of this type, hydroisomerization of the wax takes place to form high VI iso-paraffins of low pour point. In processes of this kind, the catalyst is typically a bifunctional catalyst containing a metal hydrogenation component on an amorphous acidic support. The metal component is usually a combination of base metals, with one metal selected from the iron group (Group VIII) and one metal from Group VIB of the Periodic Table, for example, nickel in combination with molybdenum or tungsten. Modifiers such as phosphorus or boron may be present. The activity of the catalyst may be increased by the use of fluorine, either by incorporation into the catalyst during its preparation in the form of a suitable fluorine compound or by in situ fluoriding during the operation of the process.
Other processes useful in the production of synthetic lubricants employ oligomerization with a Lewis acid catalyst such as promoted BF3 or AlCl3. Such processes are described in U.S. Pat. Nos. 5,136,118 and 5,146,022, the contents of which are hereby incorporated by reference for such details.
Specific automotive engine lubricant oil formulations, such as 10W-30 engine oil, have required the use of specific lubricant base stock in order to provide the requisite viscosity, lubricity, high viscosity index and other properties. In turn, the production of the specific lube base stock has been locked into certain raw materials and processes capable of producing lube stock with the requisite properties.
Products prepared from the Fischer-Tropsch process comprise a mixture of various solid, liquid, and gaseous hydrocarbons. Those Fischer-Tropsch products which boil within the range of lubricating base oil and diesel are usually of higher value than lower boiling products, such as naphtha, or normally gaseous products, such as LPG. As may be appreciated, it is advantageous to capture the carbon value of the lower boiling and normally gaseous products by upgrading them to higher molecular weight and higher value products.
Lubricating base oils may be prepared from the Fischer-Tropsch wax recovered as one of the products of the Fischer-Tropsch synthesis. Lubricating base oils typically will have an initial boiling point above 315° C. (600° F.). Accordingly, it is desirable to be able to maximize the yields of such higher value hydrocarbon products which boil within the range of lubricating base oils.
Fischer-Tropsch products, as they are initially recovered from the Fischer-Tropsch reactor, contain varying amounts of olefins depending upon the type of Fischer-Tropsch operation employed. In addition, the crude Fischer-Tropsch product also contains a certain amount of oxygenated hydrocarbons, especially alcohols, which have been reported to act as a poison to most oligomerization catalysts. To address this issue, it has been proposed to remove these oxygenates through a hydrotreating step, or, alternatively, converting them to olefins by a dehydration step. The olefins originally present in the Fischer-Tropsch products or derived from converted oxygenates may be oligomerized to yield hydrocarbons having a higher molecular weight than the original feed.
As may be appreciated, there is a continuing need for automotive lubricants which are based on fluids of high viscosity index and which are stable under the high temperature, high shear rate conditions encountered in modern engines.
U.S. Pat. No. 5,136,118 proposes a process is disclosed for the production of synthetic hydrocarbon lubricants having high viscosity index by oligomerizing a mixture of alpha-olefins comprising the reaction product from the thermal cracking of refined wax. The oligomerization is carried out with Lewis acid catalyst or reduced chromium oxide on porous support.
U.S. Pat. No. 5,146,022 proposes a process is disclosed for the production of synthetic lubricants having high viscosity index and thermal stability by oligomerizing a mixture of C5-C18 or alpha-olefins produced from the thermal cracking of slack wax or recycled slack wax. The oligomerization is carried out with Lewis acid catalyst. In one form, promoted aluminum chloride may be used as the catalyst.
U.S. Pat. No. 5,208,403 proposes lubricant compositions that comprise blends or mixtures of low viscosity, HVI lube basestock with higher viscosity, HVI PAO lube basestock produced from slack wax by thermal cracking to alpha olefins followed by Lewis acid catalyzed oligomerization of the mixture to a lube base stock. Blending these components in appropriate proportions produces lube basestock having viscosities in the range of 8-15 cS (100° C.) from which material suitable for the formulation of 10W-30 automobile engine lube can be produced. The blends are said to possess VI values greater than that of either component of the blend.
U.S. Pat. No. 5,276,229 proposes a process for the production of synthetic lubricants having high viscosity index by oligomerizing a mixture of alpha-olefins produced from the thermal cracking of slack wax or recycled slack wax. The oligomerization is carried out with a reduced metal oxide catalyst, such as a carbon monoxide reduced chromium oxide on a silica support. The olefin product from the thermal cracking step is purified by purification steps, such as adsorption of impurities by molecular sieve or oxygenates removal catalysts, or by selectively hydrotreating to remove any dienes prior to the oligomerization step.
U.S. Pat. No. 6,700,027 proposes a process for increasing the yield of C10 plus hydrocarbon products from a Fischer-Tropsch plant which comprises recovering a Fischer-Tropsch condensate fraction boiling below 70° F. from the Fischer-Tropsch plant, wherein said fraction contains at least 10 weight percent or more olefins, contacting the olefins in the Fischer-Tropsch condensate fraction under oligomerization conditions, at a reaction temperature between 650° F. and 800° F. with an oligomerization catalyst comprising active chromium on an inert support and recovering a C10 plus hydrocarbon product.
U.S. Patent Publication No. 2005/0148806 proposes a method for the preparation of lower olefins by steam cracking, wherein the feed containing heavy hydrocarbons obtained by Fischer-Tropsch synthesis is subjected to steam cracking in a naphtha-designed steam cracking furnace for steam cracking the Fischer-Tropsch hydrocarbons into the lower olefins.
Many of the prior patents and literature teach the presence of large amount of oxygenated compounds in Fischer-Tropsch wax products. These oxygenates may interfere with the cracking process and if they survive the cracking conditions, they may interfere with the polymerization steps.
Despite these advances in the art, there is a continuing need for a process for preparing poly alpha olefins and lubricant base stocks from Fisher-Tropsch products.